Illumos 5810 - zdb should print details of bpobj

Illumos 5810 - zdb should print details of bpobj

5810 zdb should print details of bpobj
Reviewed by: Prakash Surya <prakash.surya@delphix.com>
Reviewed by: Alex Reece <alex@delphix.com>
Reviewed by: George Wilson <george@delphix.com>
Reviewed by: Will Andrews <will@freebsd.org>
Reviewed by: Simon Klinkert <simon.klinkert@gmail.com>
Approved by: Gordon Ross <gwr@nexenta.com>

References:
  https://www.illumos.org/issues/5810
  https://github.com/illumos/illumos-gate/commit/732885fc

Ported-by: DHE <git@dehacked.net>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3387